Collarene

In supramolecular chemistry [31] iterative synthesis is frequently used for the construction of ribbon and belt shaped molecules [32]. The repeated Diels-Alder reaction is suitable for the preparation of beltenes, collarenes and cycla-cenes containing partially or fully unsaturated annelated six-membered rings. They are potential molecular receptors for appropriate substrates. Stoddart et al. synthesized the cyclacene 18 by Diels-Alder cyclization of the bisdiene 15 and the bisdienophil 16 [33] (Fig. 10). 

Fig. 16. Apart from being an appealing target in its own right, kohnkene 43 represents a potential precursor to a wide range of extremely novel cyclacene derivatives such as 49, 50, and 51, and saturated hydrocarbons, such as 52. To date, [12]collarene 49 marks the extent to which further synthetic elaboration [122] has progressed in our laboratories...

Scheme 11. The synthetic route to [12]collarene 49 progresses via deoxygenation of kohnkene 43 to afford dideoxykohnkene 45. Acid-catalyzed dehydration of 45 gives a mixture of isomeric hydrocarbons, including presumably 46, from which 47 is isolated as the major product (56%) [21, 122]. A Birch reduction of 47 ceases upon the production of the highly-stable [12]collarene structure 49...

Figure 11. Structures of (a) collarenes, (b) cyclacenes, (c) beltenes, and (d) [8]beltene complexed to the rubidium cation.

The following Birch reduction, aimed at synthesizing [12]beltene, resulted in the production of a symmetrical dodecahydro[12]cyclacene with the probable constitution 10 called [12]collarene by Stoddart. This structure with six isolated benzene rings is believed to be the most stable compound along the route to... 

The expenditure for the synthesis of 13 can be realized by the fact that the starting material 11 is obtainable in only 0.6% yield by reaction of hexabromoben-zene with furan. Nevertheless the extension of the synthetic strategy of collarenes to macropolycyclic skeletons is a fascinating challenge. 

Key words. Cavitands, Diels-Alder reactions, stereoelectronic control, kohnkene, clathrate formation, chemical sensor, [12]collarene, [12]beltene, [12]cyclacene, organic zeolites. 

Structural formulae of collarene maerotyeles. The [injparacyclophane backbone of pillararenes - contained within the eollarene framework -is illustrated in black, while the additional bridging methylene units are illustrated in gray. 

Figure 12.6 Graphical illustration of the DFT (B3LYP/6-31G ) frontier molecular orbitals of the [12]collarene molecular belt 12.18b.

The synthesis (Scheme 12.8) of the first [l2]collarene molecular belt (12.18b) was achieved in our laboratory through the repeated application of Diels-Alder (DA) cycloadditions between the dienophile 12.20 and the diene 12.21, which proceeds with extraordinarily high diastereoselectivity. While the DA reaction between 12.20 and 12.21 could, potentially, lead to four different diastereoisomeric products, only a single diastereoisomer was actually observed in this reaction. This especially high diastereoselectivity is dictated primarily by electronic effects, which kinetically favor attack on the diene 12.21 from the endo face, while attack on the dieneophile 12.20 is preferred from the exo side. Density functional theoiy (DFT) calculations (Figure 12.7) reveal that the observed diastereoselectivity has its cause in the selective stabilization of the endo-exo transition state (12.19a). 

Figure 12.7 All four possible transition states of the highly diastereoselective Diels-Alder (DA) cycloaddition reaction employed for the synthesis of the [12]collarene 12.18b. The superb diastereoselectivity of the reaction is explained by the DFT (B3LYP/6-31G ) transition-state energies, which have been plotted relative to the energy of the starting materials.

Collarenes for Selective Cation Binding and Membrane Transport... 

---